This invention relates to a novel technique, system and apparatus for preventing the occurrence of moire on the screen of a cathode ray tube (CRT).
Moire is a shimmering optical effect which will be observed when two similar repetitive structures are aligned at critical angles and exposed to light. The occurrence of moire is a definite problem with a CRT where the repetitive structures are, on the one hand, lines constituted by openings in the shadow mask of the CRT and, on the other hand, lines constituted by the raster scanned by the electron beam of the CRT.
Modern television receivers and monitors employ tracking focus in order to optimize electron beam spot size. More specifically, as the beam current of a CRT varies, it is necessary, in order to maintain proper focus, to correspondingly vary the spot size of the electron beam. Electron optics used in modern CRT's produce optimum spot size at all operating conditions (brightness variations) when the potential of the focus electrode is maintained at between about 18-20% of the potential of the high voltage anode. The maintenance of this condition with beam current variations is the function of the tracking focus network. However, it is this network which also is responsible for moire. More specifically, the function of the tracking focus network is to vary the DC potential of the focus electrode in response to beam current variations, thereby varying electron beam spot size and maintaining the electron beam focused. At some critical point for each cathode ray tube moire will occur when the spot size is reduced to some specific size relative to the aperture spacing (between horizontal lines) of the shadow mask of the CRT. Generally speaking, moire will occur when the ratio of the dimension of a raster line of video information to the aperture spacing of the shadow mask is about 2:1 or less. Thus, if any given CRT is initially properly focused, and tracking focus is employed, the beam current of the CRT can increase to a point where operation of the tracking focus network will result in a spot size small enough to cause moire.
A currently accepted technique to overcome the problem of moire is to defocus the electron beam under all operating conditions. This prevents the spot size from ever becoming so small as to cause moire. Obviously, however, picture resolution is poor because the picture is defocused at all times.